In the wake of the ongoing deregulations of the electric power markets, load transmission and wheeling of power from distant generators to local consumers has become common practice. As a consequence of the competition between utilities and the emerging need to optimize assets, substantially increased amounts of power are transmitted through the existing networks, occasionally causing congestion, transmission bottlenecks and/or oscillations of parts of the power transmission systems. In this regard, electrical transmission networks are highly dynamic, and in response to changing network states, loads or power injected by generating units, the power flow over alternate transmission paths may need to be redistributed.
Therefore, a network based control for a redirection and a uniform redistribution of power flows in the transmission system without generation rescheduling or topological changes becomes a very important instrument in the hands of independent Transmission System Operators (TSO). Changes are made according to the current topology and electrical flow situation of the electrical transmission network, e.g., by means of network controllers or Power Flow Control Devices (PFC) that are used to control the bus voltages, line currents or phase angles and that are designed to supply reactive power to support voltage and provide stability enhancements. These devices are installed at transmission line stations to adjust power flow in each transmission line, so that power can be guided to flow in a safe, stable and balanced manner in a large number of lines within the electrical transmission network.
PFCs such as a Phase Shifting Transformer (PST), a Flexible Alternating Current Transmission Systems (FACTS) device or a High Voltage Direct Current (HVDC) device improve dynamic performance of electrical transmission networks. Examples of FACTS devices that can perform power flow control are the Unified Power Flow Controller (UPFC), the Static Synchronous Series Compensator (SSSC), the Thyristor Controlled Series Compensator (TCSC) and the Thyristor Switched Series Compensator (TSSC). PFCs are controlled via control or operational parameters, resulting in either discrete (e.g. PST and TSSC) or continuous responses from the PFC devices depending on their nature.
The conventional practice is to change the control or operational parameters of the PFCs in a rather static way, and/or use a closed loop control based on local measurements performed in the substation where the PFC device is installed. For example, in the case of a PST the position of the tap changer is computed based on global loss minimization or optimal power flow calculations taking transfer limits of various components into account, and the position manually set by an operator and typically updated on a time-scale of hours. This way transfer limits also on other paths than those under direct control can be taken into account. Typical operation practices imply that the PFCs are set such that transfer limits are satisfied without additional power flow control effort also if any one network component is disconnected due to fault. This security constraint, which often is referred to as the N−1 security constraint, constitutes a trade off between efficient operation of the transmission network and the security of the network in case of disturbances. Thus in the time period prior to a disturbance the network is operated less efficiently due to the security constraint. Furthermore, in the event of more severe disturbances, i.e. disturbances not included in the security constraint, the response is slow since there is an operator in the loop and the state snapshots, which often are obtained through a SCADA system and state estimation, may not be updated reliably or rapidly enough to make sure transfer limits are complied with. In turn, this may lead to cascaded line tripping as overloaded lines are disconnected by local protection, if the combined response time of the SCADA/EMS system and the operators is too long.
Fast network controllers or Power Flow Control Devices (PFC) which are based on power electronic semiconductor components and which do not rely on mechanical switches for their main functionality, enable response times in the millisecond range. They include, among others, the aforementioned Flexible Alternating Current Transmission System (FACTS) devices as well as High Voltage DC (HVDC) devices. HVDC devices comprise line commutated converters or voltage source converters for rectifying AC active power to DC power and inverting DC power back to AC active power, which converters are based on a multitude of semiconductor components or modules that are individually controlled by control signals produced by gate drives or other control hardware of a converter controller.
By way of example, the primary controllers that are embedded in FACTS devices are typically of P- or PI-type, with occasional supplementary controllers like damping controllers. Normally, the set-points for FACTS devices are kept constant or changed manually on a slow time scale based on market activities or optimal power-flow calculations. Typical FACTS device controllers operate purely based on rather simple local objectives such as keeping constant, or as close as possible to a specified reference value, the power flow on a certain line or the voltage in one point in the network, or improving a transfer capability of transmission corridors.
With such a fast but local control, a controlled path can be protected from overloads since flows can be diverted by the PFC, however, the effects on other parts of the network are not taken into account. In disturbance cases, local control on certain paths can contribute to overload and tripping of other circuits with cascaded line tripping as a result. For this reason, power flow control can have a detrimental effect on the overall system stability, and power utilities are therefore cautious when equipping PFC with automatic control.
A state or condition of an electric power system at one specific point in time can be obtained from a plurality of synchronized phasor measurements or snapshots collected across the electric power system or power transmission network. Phasors are time-stamped, complex values such as amplitude and phase, of local electric quantities such as currents, voltages and load flows, and can be provided by means of stand-alone Phasor Measurement Units (PMU). These units involve a very accurate global time reference, obtained e.g. by using the Global Positioning Satellite (GPS) system or any other comparable means, and allowing synchronization of the time-stamped values from different locations. The phasors are sampled at a rate of 20 to 60 Hz, and thus can provide a view on transient or sub-transient states that goes beyond the rather static view as provided by SCADA/EMS. Conventionally, PMUs forward their measured phasor values to a system protection centre at control level or alternatively to a PMU acting as a master. Data exchange can further be established between the system protection centre and other control and protection systems to allow for optimal data sharing and control actions based on oscillation detection and frequency deviations.
EP1134867 discloses a method that assesses the stability of an electric power transmission network. It comprises measuring voltages and currents at a plurality of locations of the network, transmitting the latter as well as information regarding the state of switches of at least one substation to the system protection centre, and generating at least one stability margin value of the transmission network there from. In this way, detailed real-time information about the state of the network is collected at a system level of the network, allowing a corresponding global analysis of the information.
Due to the physical laws governing the flows in the power transmission network or electricity grid, the power flows distribute according to a “law of least resistance”. A consequence of this is that not all components in the network reach their limits (which may be thermal overload, or limits based on other considerations such as voltage or transient stability) simultaneously. Therefore, increased utilization of the grid can be allowed if flows are diverted from lines that are overloaded onto lines with higher thermal margins or stability limits.
In the article by M. Larsson et al. “Improvement of Cross-border Trading Capabilities through Wide-area Control of FACTS”, Proceedings of Bulk Power System Dynamics and Control VI, 22-27 August, Cortina D'Ampezzo, Italy, 2004, coordination of a multitude of FACTS devices is proposed. A secondary control loop generates the set-points for the primary FACTS controllers, based on global or wide-area information. The latter comprises state snapshots from a wide-area measurement system including a relatively large number of Phasor Measurement Units (PMUs). The subsequent mathematical optimization of the FACTS set-points occurs in real time with respect to e.g. an avoidance of overloading corridors, controlling power flows to a predefined reference, voltage security assessments and/or accurate stability margins. The design of a secondary, wide-area controller logic relies on a detailed inspection of the network topology and a prioritization of various control objectives. A computationally expensive optimization procedure based on information related to topology and system state has to be executed at least at each update of the underlying information.